ERROR stack trace

java.lang.NullPointerException
at org.broadinstitute.sting.gatk.walkers.variantrecalibration.VariantRecalibratorEngine.generateModel(VariantRecalibratorEngine.java:83)
at org.broadinstitute.sting.gatk.walkers.variantrecalibration.VariantRecalibrator.onTraversalDone(VariantRecalibrator.java:359)
at org.broadinstitute.sting.gatk.walkers.variantrecalibration.VariantRecalibrator.onTraversalDone(VariantRecalibrator.java:139)
at org.broadinstitute.sting.gatk.executive.Accumulator$StandardAccumulator.finishTraversal(Accumulator.java:129)
at org.broadinstitute.sting.gatk.executive.LinearMicroScheduler.execute(LinearMicroScheduler.java:116)
at org.broadinstitute.sting.gatk.GenomeAnalysisEngine.execute(GenomeAnalysisEngine.java:313)
at org.broadinstitute.sting.gatk.CommandLineExecutable.execute(CommandLineExecutable.java:113)
at org.broadinstitute.sting.commandline.CommandLineProgram.start(CommandLineProgram.java:245)
at org.broadinstitute.sting.commandline.CommandLineProgram.start(CommandLineProgram.java:152)
at org.broadinstitute.sting.gatk.CommandLineGATK.main(CommandLineGATK.java:91)

ERROR A GATK RUNTIME ERROR has occurred (version 2.8-1-g932cd3a):

ERROR

ERROR MESSAGE: Code exception (see stack trace for error itself)

I have read similar problems on GATK forums and based on those, it seems to me that the training set of VCFs is too small for my data. Is that so? If so, can you please tell me how can I fix it? This is for rice and I only have 1 set of known VCFs from dbSNP.

Can you also please confirm if my settings for 'known', 'training' and 'truth' are correct.

I'm trying to run GATK with the rice genome and I'm having trouble finding a known variant list of rice? Does anyone have a link or a more general list of known variant resources for other reference genomes?
Specifically, I found two rice genomes in dbSNP, but I can't find any documentation about which is which, and once I'm in the FTP site, which file I should be using?
ftp://ftp.ncbi.nih.gov/snp/organisms/rice_4530/
ftp://ftp.ncbi.nih.gov/snp/organisms/rice_4536/
Maybe take all the VCF files for each chromosome and cat them into one big file? Clearly, I'm a little clueless here :)
ftp://ftp.ncbi.nih.gov/snp/organisms/rice_4530/VCF/